Because of their enlarged measuring range of 360° and enlarged signal amplitudes and thus lower susceptibility to interference compared to AMR angle sensors, magnetoresistive sensor elements based on the GMR (giant magneto resistance) effect according to the so-called spin valve principle are increasingly being utilized for angle detection in motor vehicles. To that end, the sensor system has a field-generating magnet and an angle sensor, or more generally a magnetoresistive sensor element, positioned in proximity thereto, the direction of the magnetic field acting on this sensor element being detected.
The design of magnetoresistive sensor elements on the basis of the spin valve principle differs from GMR sensor elements or GMR multilayers, which have an alternating sequence of ferromagnetic and non-magnetic thin layers, to the effect that as a rule, only two ferromagnetic thin layers are provided that are separated by a non-magnetic intermediate layer. The direction of the magnetization of one of these two ferromagnetic layers is then fixed (pinned) by a coupling of this layer to an anti-ferromagnetic layer. The other layer, the so-called free layer or detection layer, in an outer magnetic field is able to freely rotate the direction of its magnetization, so that an angle, variable via the direction of the outer magnetic field, sets in between the direction of the magnetization in the detection layer and the direction of the magnetization in the pinned layer, i.e., the so-called reference layer. The electrical resistance of the sensor element is moreover a function of this angle, so that this angle can be determined by measuring the electrical resistance.
Alternatively, a layer system having an anti-ferromagnetic layer, a ferromagnetic layer situated thereon, a non-magnetic layer on it, and a ferromagnetic layer thereon, is also suitable for fixing the direction of the magnetization of the reference layer, the non-magnetic layer between the two ferromagnetic layers imparting an anti-ferromagnetic coupling between them. Such a layer system is known as an artificial anti-ferromagnet.
To permit implementation of an angle sensor having magnetoresistive layer systems operating on the basis of the GMR effect and constructed according to the spin valve principle, it is beneficial to interconnect a plurality of such magnetoresistive layer systems in two Wheatstone bridge circuits, one bridge being rotated by 90°, for example, compared to the other with respect to the direction of the magnetization in the reference layer. In an outer rotating magnetic field, for instance, this leads to a phase shift in the output signals of both bridges. One speaks here of a “cosine” bridge and a “sine” bridge according to the dependence of the output signal of the two bridges on the outer magnetic-field direction. Each of the individual Wheatstone bridges is further made up of magnetoresistive layer systems in the form of four single resistors, which pairwise have a direction of magnetization in the reference layer of, for example, 180° relative to each other.
Published international patent document WO 00/79298 provides an overview of the design of magnetoresistive layer systems that are constructed according to the spin valve principle and operate according to the GMR effect. Also described in this patent document is the interconnection of magnetoresistive layer systems in the form of two Wheatstone bridge circuits rotated by 90° relative to each other, as well as the structure of a magnetoresistive layer system having an artificial anti-ferromagnet for setting a fixed direction of the magnetization of the reference layer of the layer system.
Measuring errors due to two intrinsic effects occur when working with magnetoresistive layer systems that operate on the basis of the GMR effect and are constructed according to the spin valve principle.
Thus, the detection layer or free layer has a certain anisotropy on one hand, and on the other hand, a residual coupling with the reference layer or pinned layer, i.e., it does not optimally follow the outer magnetic field with respect to its direction. Moreover, one also observes that the direction of the magnetization of the reference layer does not remain completely unchanged when an outer magnetic field acts on it. In this respect, a slight change in the direction of the magnetization of the reference layer also frequently occurs in response to the influence of an outer magnetic field, which invalidates the measuring result.
An object of the present invention is to provide a magnetoresistive sensor element having a reduced angle error compared to the known art, and a method for reducing the angle error of a magnetoresistive sensor element, to thus permit use of this magnetoresistive sensor element as a particularly precise angle sensor, e.g., in motor vehicles.